The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
The exterior of a vehicle generally includes multiple layers of paint and/or other coatings, such as electrocoat, primer, basecoat, and clear coat. Each layer has a minimum film build designed to inhibit the degradation and potential delamination of the exterior due to, for example, UV and visible light, and to provide the appropriate appearance/color to the vehicle, and to protect the substrate from environmental damage.
While various paint thickness measurement techniques are available for measuring a single layer of paint, there are a limited number of non-destructive measurement techniques for measuring multiple layers. One such measurement technique utilizes ultrasound technology in which an ultrasonic transducer is placed on the exterior surface, and sends an ultrasonic signal through the exterior surface. A liquid couplant, usually water, is used to transmit the signal into the coating material. The ultrasonic signal generates an echo at the layer interfaces, and the thickness is determined based on the time difference between the successive echoes. Sound velocity values vary among the different coatings, so calibration is performed on all layers in addition to the various basecoat colors.
While the ultrasound technique is effective, there are some issues with this technique. For example, the transducer size and the tool used for the transducer may not allow measurement of certain vehicle surfaces, such as a windshield flange, and thus, a separate procedure is usually employed to obtain data of those areas. Another issue is that the transducer requires a large (e.g., 10 mm diameter) flat area in order to generate adequate waveforms. This requires selecting points on a vehicle based on their flatness rather than being able to select locations on the vehicle that are of interest but may not be flat. Furthermore, the transducer physically contacts the vehicle. Although damage from the transducer may not occur, the water left on the body affects other quality control measures, such as a dirt detection quality check.
Another technique for measuring a multi-layer surface includes the use of a radiation beam having a terahertz (THz) frequency. Using a THz light source to generate a THz radiation beam, a THz head is positioned at a designated offset and is normal to a target surface of the vehicle before the measurement is performed. For example, the radiation head can be attached to a robot or some other piece of automation to allow it to track the contour surfaces and the complex geometries. The THz radiation beam is emitted from the Thz radiation head and reflects off the vehicle due to a change in refractive index. The time difference between the emission and reflection is used to calculate the thickness. The THz signal reflects off of the coating interfaces, due to a change in refractive index, and the time difference of the reflection is used to calculate the thickness.
Since the THz head is fairly compact and does not contact the surface of the vehicle, it can be used to measure places not measurable by an ultrasonic transducer, such as the windshield flange. The THz radiation beam is typically 1 mm in diameter which enables measurement of multiple regions that have a flat section of that size.
However, for an optimal measurement, the THz radiation head should be aligned normal to a target surface of the vehicle so that the emitter of the radiation head aligns with the detector of the radiation head. When the emitter and detector are aligned, the amplitude of the reflected radiation signal is usually at the maximum value. If the radiation head is not normal to the surface, the reflected radiation signal may not align with the detectors, which results in a lower peak amplitude. This loss in signal may affect the results of the thickness measurement. Misalignment during the calibration procedure would also result in an incorrect calibration file and bad data. These and other issues are addressed by the teachings of the present disclosure.